The two important things that we look at are the surface morphology (AFM or STM),
and the terminating atomic layer composition (CAICISS). The AFM image in Fig. 1
shows a rather typical as-received wet-etched SrTiO3 surface. The terrace width depends
on the miscut angle and is about 230 nm in this case.

CAICISS measurements at room temperature or up to about 250°C show only a Ti peak
along the 111 direction He-ion scattering (Fig. 2), which means that the surface
is terminated only by the TiO2 layer. Upn heating, a Sr peak starts to
appear at above 250°C and remains constant at 500°C and above.
The peak intensity corresponds to a Sr coverage of about 15%. This means that
the wet-etched SrTiO3 surface is not stable at high temperature and Sr segregation
will always occur when the substrate is heated to typical thin film growth temperatures.

Room temperature, after annealing at 550°C

Fig. 3: SrTiO3 STM, 25 × 25 nm2

Fig. 4: SrTiO3 STM, 100 × 100 nm2

Fig. 5: SrTiO3 STM, 200 × 200 nm2

These images were taken at room temperature after keeping a substrate at 550°C
for several hours. Unit cell layers can still be seen (black, gray and white in
Fig. 4, but the terraces are broken into meandering islands. It is hard to
identify where the 'step edges' are. This breakup coincides with the increase of
Sr detected by CAICISS. Despite the breakup of the surface in the lateral direction,
all the steps and small islands still have a uniform height of a single STO
unit cell. On a finer, 25 nm scale, the island structure is clearly visible. These
islands have a diameter of 5 to 10 unit cells (2 to 4 nm). High-temperature annealing
is therefore necessar to recover continuous terraces before deposition.

Annealing at 600°C

Fig. 6: SrTiO3 STM at 600°C, 100 × 100 nm2

Fig. 7: SrTiO3 STM at 635°C, 110 × 110 nm2

The surface continues to evolve at higher temperatures. Smaller islands start
to combine with neighboring islands, forming terraces that still have many holes.
Interestingly, the first feature to form is a continuous step edge.
This may indicate that surface atom diffusion along a step edge is faster than
across a terrace. This would explain why
holes in a lower layer are rapidly filled close to a step edge in the upper layer.
The surface migration behavior shows no indication of diffusin barriers
occurring on step edges and material from higher layers feeds the filling of
holes in the lower layers.

The surface morphology changes dramatically when the temperature is between 600 and 700°C.
Uniform terraces start to reappear and the island size increases. Some of the local
island edges appear to follow the [100] or [010] directions marged with white arrows in
Fig. 7. Despite the faceting, the average step edge direction is still determined
by the miscut direction.

Annealing at 700 to 800°C

Fig. 8: SrTiO3 STM at 700°C, 110 × 110 nm2

Step edge movement starts to be clearly visible at around 700°C and can be followed
in real time by high-temperature STM. Small holes are no longer present on the terraces.
Only larger features remain, but even the remaining holes are gradually being filled
by atoms migratng on the terraces. The migration rate at 700°C is fast enough
to be directly imaged by high-temperature STM.

A SrTiO3 substrate was heated in small steps of 10 to 20°C at a time. The temperature
was allowed to stabilize for about a minute and a series of scans was taken at each
temperature step.
The animations were produced by cutting regions from
the STM scans and aligning them with each other. This way the
thermal drift of the imaging area could be compensated. Unfortunately
this also reduced the size of the images. Animations are available in
both AVI and QuickTime formats.

Annealing at 800°C

Fig. 9: SrTiO3 STM at 800°C, 240 × 240 nm2

Perfectly straight step edges are finally back! The terrace width of this substrate
was fairly small, around 25 nm. For wider terraces either the heating time must be much
longer or the temperature higher. For substrates that have no intentional miscut,
the terrace width can be very large, requiring ~2h annealing times, for a 0.2° miscut
substrate 15 minutes at 950°C seems to do the trick. At these temperatures the step
edges move very fast. The last anymation shows a particle on the surface,
pinning the step edge. The step edge can shift by more than a nm between frames (10 seconds).
The edge oscillates around its equilibrium position, which
shows that the movement is not caused by evaporation, but by rapid surface diffusion.